1. Field of the Invention
The present invention relates to integrated circuits, and, in particular, to circuits having switched capacitors.
2. Description of the Related Art
FIG. 1 shows a circuit diagram of a conventional ladder filter structure. The ladder structure comprises shunt resonant LC tanks implemented with amplifiers, capacitors, and resistors, which are then coupled by series coupling capacitors C.sub.1, C.sub.2, . . . .
FIGS. 2(A) and 2(B) show circuit diagrams of conventional single-ended and differential implementations, respectively, of the +/-R resistive elements in the ladder filter of FIG. 1. These implementations, which consist of one or two discrete capacitors C.sub.R configured with sets of switches S.sub.i, are referred to as switched capacitors. The relative phasing of the opening and closing of the various switches (as controlled by a clock signal) dictates whether the switched capacitor implements a +R element or a -R element.
For a given capacitance value C.sub.R and a given clock frequency, the ladder filter of FIG. 1 will have a specific fixed frequency response. When implemented with discrete elements, the capacitance value C.sub.R will be fixed for a given implementation and, as a result, the only way to change the frequency response of the ladder filter is to change the clock frequency. Unfortunately, most integrated circuits have only a finite number of discrete clock frequencies available. As such, the number of different frequency responses for the ladder filter are also limited to a finite number of discrete responses.